The Llano Estacado region of western Texas and adjacent New Mexico has one of
the most species-rich charophyte floras known for North America, but upon
closer inspection this flora is seen to consist of two, strictly
non-overlapping segments, one of 12 taxa, the other of nine. The larger group
of species is confined to a vast series of shallow, ephemerally inundated
depressions known as playas. These normally fill with run-off rainwater in
late May or early June, remain flooded through early to mid-September before
eventually drying to the curling polygon stage near the end of September.
Approximately 8 months later the cycle is repeated. The remaining nine
charophytes are confined to permanently inundated sites, the most common of
which are livestock-watering windmill complexes, known as papalotes. Rarely,
if ever, are members of the playa-12 encountered in papalotes or members of
the papalote-9 in playas. The underlying bases for this ‘two
flora’ dichotomy stem from the contrasting amphipod populations
engendered by the two habitat types. Most freshwater amphipods (scuds),
including the common North American herbivore
Hyalella azteca (Saussure), cannot withstand complete
desiccation which, accordingly, prevents its colonisation of playas. By
contrast, scuds often reach exceptionally high densities in the associated
concrete or metal tanques of papalotes, since such specialised habitats are
usually free of vertebrate predators, e.g. birds, fish, turtles, salamanders.
Scud- resistant charophytes are larger and slower to develop gametangia,
features inimical to success in short- lived playas. By contrast, precocious
charophyte opportunists–typical of playa floras–cannot withstand
the grazing pressures of papalote (or papalote-like) environments. While
charophytivory is assumed to be of worldwide occurrence and significance, it
has been clearly demonstrated only in the unique juxtaposition of
‘playas y papalotes’.